dungeon_overview adjustment

The dungeon_overview bits in the rm structure were being
clobbered by a run-length encoding save/restore because
they weren't taken into consideration.

This patch pulls that data out of the rm structure completely.

It also adjusts the run-length encoding checks to take the
candig bit into consideration and adds a comment to rm.h
reminding people to make run-length encoding adjustments
in save.c for any new bits that get added.

src/bones.c

@@ -402,7 +402,7 @@ struct obj *corpse;
 	    levl[x][y].waslit = 0;
 	    levl[x][y].glyph = cmap_to_glyph(S_stone);
 #ifdef DUNGEON_OVERVIEW
-	    levl[x][y].styp = 0;
+	    lastseentyp[x][y] = 0;
 #endif
 	}